1. Field of the Invention
The present invention relates to a fully integrated linear regulator with a Darlington bipolar output stage that may be suitable for applications such as a floating power supply for lead batteries.
2. Description of the Related Art
It is known that regulators of the fixed type, i.e., with a preset output voltage, are commonly used and are aimed at applications, such as a floating power supply for lead batteries usable for example within uninterruptible power supplies for electronic computers and the like.
FIG. 1 is an exemplary diagram of a potential application of a conventional linear regulator used as a floating power supply for a lead battery. The linear regulator 1 is connected to terminals 2 of a power supply with a transformer 3 and a rectifying and filtering means 4 interposed therebetween.
The linear regulator 1 has an input voltage terminal 5 and output voltage terminal 6. An output voltage Vout on the output voltage terminal 6 of the regulator 1 is set by a pair of resistors RA and RB and is capable of being fine-adjusted during production by a trim resistor TR1. At the output voltage terminal 6 of the regulator 1, there is also a diode D5 designed to prevent, when an input voltage Vin on input voltage terminal 5 is floating, battery 7 from discharging across the pair of resistors RA and RB and across the regulator 1.
FIG. 2 is a block diagram of the linear regulator 1a with an output stage provided by an NPN Darlington transistor configuration. In particular, FIG. 2 is a view of a situation in which the input voltage Vin on input voltage terminal 5 of the linear regulator 1a is left to float because the main power supply is not present.
In this floating input voltage Vin situation, the emitter-base junction of the NPN transistor Q3 sees a voltage that is higher than the breakdown voltage of the NPN transistor Q3 and undergoes a Zener breakdown. When the Zener breakdown occurs, a forward-biased base-collector junction results and a DC path toward starting circuit 8 is formed. At this point, the starting circuit 8 operates to activate current sources Iref1, Iref2 and Iref3, which begin to draw more current to the collector of the transistor Q3, thus further increasing reverse current Irev discharged from the battery 7.
The presence of the diode D5 arranged at the output of the linear regulator 1 greatly limits the reverse current Irev. However, it should be noted that such a circuit solution of the discrete type has some drawbacks.
First of all, there is the drawback of a high cost, which is due basically to the presence of the trim resistor TR1 and of the diode D5 arranged at the output voltage terminal 6 of the linear regulator 1 (see FIG. 1), which must be sized in order to withstand all the charging current of the battery 7. Moreover, there is a cost due to the time required to adjust the output voltage Vout.
The electrical performance of the conventional linear regulator is degraded because, from a thermal standpoint, the presence of the diode D5 arranged at the output voltage terminal 6 of the linear regulator 1 significantly degrades the thermal coefficient of the output voltage terminal 6 of the linear regulator 1 and also drastically degrades load regulation in the operable range of the output current. Integration of the diode D5 in silicon, in addition to the resistors RA and RB, is not convenient, both due to the above-mentioned problems and because an enormous increase in silicon area would be required due to the high current capacity required.
On the other hand, providing integrated diodes with a blocking capacity that is higher than the intended output voltage Vout on output voltage terminal 6 can be difficult. This is so because if it is necessary to use a low-cost bipolar type diode, the available diodes have base-emitter junctions that can only withstand, in reverse mode, an output voltage Vout that is lower than the intended output voltage Vout on output voltage terminal 6 of the linear regulator 1.
The present invention includes a linear regulator with a Darlington bipolar output stage. The linear regulator includes a starting circuit, an output stage, and a reference voltage generator connected to a control loop. The starting circuit has output terminals connected to current sources and an input terminal connected to an input reference terminal of the linear regulator via a transistor of the PNP-type. The output stage includes two Darlington-connected transistors, and an emitter terminal of the second of the two Darlington-connected transistors being connected to the output voltage terminal of the linear amplifier. The reference voltage generator supplies a voltage value approximately equal to the chosen output voltage value of the linear regulator. The control loop is configured as a voltage follower and includes an error amplifier, the positive input terminal of the error amplifier being connected to the reference voltage generator, the inverting input terminal of the error amplifier being connected to the output voltage terminal of the linear regulator, and the output terminal of the error amplifier being connected to the output stage.
In operation, the linear regulator provides a regulated voltage to a battery terminal while the input voltage terminal of the linear regulator receives an input voltage from a main power supply. If the input voltage terminal of the linear regulator fails to receive the input voltage from the main power supply, the voltage regulator stops regulating, essentially, and isolates the battery so that the system being regulated continues to receive power from the battery rather than the linear regulator.